The present invention relates generally to wireless communication systems and, more particularly, to mobile station receiver architectures and methods that employ multiple receive chains.
Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as a client terminal or mobile station 12 and one or more base stations 14. Other network devices may also be employed, such as a mobile switching center (not shown). As illustrated, the communication path from the base station 14 (“BS”) to the client terminal or mobile station (“MS”) 12 is referred to herein as a downlink (“DL”) direction, and the communication path from the client terminal 12 to the base station 14 is referred to herein as an uplink (“UL”) direction. In some wireless communication systems, the MS 12 communicates with the BS 14 in both the DL and UL directions. For instance, such communication is carried out in cellular telephone systems. In other wireless communication systems, the client terminal communicates with the base stations in only one direction, usually the DL. Such communication may occur in applications such as paging.
FIG. 2 shows the client terminal 12 in greater detail. The client terminal/MS 12 typically includes a baseband subsystem 16 and a radio frequency (“RF”) subsystem 18. A memory unit, such as an external memory 20, may be connected to the baseband subsystem 16.
FIG. 3 shows an example of a baseband subsystem in greater detail. The baseband subsystem 16, typically includes a micro controller unit (“MCU”) 22, a signal processing unit (“SPU”) 24, data converters 26, various peripherals 28, a power management unit 30, and internal memory 32. The SPU 24 may be formed of one or more digital signal processors (“DSP”), hardware (“HW”) accelerators, co-processors, or a combination of the above. Normally, the overall control of the baseband subsystem is performed by software running on the MCU 22, and the processing of signals is carried out by the SPU 24.
Of the converters 26, analog to digital converters (“ADCs”) are provided to convert a received analog signal into a digital signal which enables the baseband subsystem to process it. Also, digital to analog converters (“DACs”) are provided to convert the processed baseband digital signal into an analog signal for transmission. The ADCs and DACs are collectively referred to herein as the “data converters.” Such data converters can either be part of the baseband subsystem or part of the RF subsystem, and depending on their location, the interface between the two subsystems will be different. The location of the data converters, however, does not alter the overall function of the client terminal.
FIG. 4 shows an example of an RF subsystem in greater detail. The RF subsystem 18 typically includes a receiver 34, a transmitter 44, a synthesizer 58, a power amplifier 52, an antenna 54, and other components. The RF subsystem 18 shown in FIG. 4 is for a time division duplex (“TDD”) system. The receiver section 34 converts the signal from RF to baseband and includes mixers 36, filters 38, low noise amplifiers (“LNAs”) 40 and variable gain amplifiers (“VGAs”) 42. The transmitter section 44 converts the baseband signal to the RF signal and includes mixers 46, filters 48, and gain control VGAs 50. Power amplification of the transmit signal is typically carried out by a separate power amplifier (“PA”) unit 52 but is considered part of the transmit RF chain. In some architectures, some of the above components of the receiver and transmitter sections are shared. The receiver section 34 and the transmitter section 44 are coupled to an antenna 54 via a transmit/receive (T/R) switch 56. The synthesizer 58 is also shown as being coupled to the receiver section and to the transmitter section.
Down conversion in the receiver and up conversion in the transmitter can be performed using a single stage or using multiple stages, each leading to different implementations of the RF subsystems. One possible implementation is known as direction conversion or zero intermediate frequency (“IF”) wherein the downlink RF signal is converted to a baseband signal by a single mixer and local oscillator (“LO”). Another implementation employs a super-heterodyne structure which uses one or more IF stages and LOs for converting the RF signal to the baseband signal. Yet another implementation uses an approach called “low IF” that converts the analog RF signal to an analog low intermediate frequency signal and then convert the analog intermediate frequency signal to a digital signal using high speed data converters.
Often, multiple receive and transmit chains are used in these wireless communication systems in order to improve their performance. Such performance improvement can be shown in terms of better coverage, higher data rates, the multiplexing of multiple users on the same channel and at the same time, or some combination of the above.
FIG. 5 illustrates an RF subsystem 60 with two RF receive chains. The RF subsystem includes a transmitter 62, a synthesizer 64, and a pair of receivers 661 and 662. One of the receivers 661 and the transmitter 62 are coupled to a first antenna 681 via a transmit/receive switch 69. The other receiver 662 is connected to a second antenna 682. Different techniques for using multiple receive and/or transmit chains are often referred to with different names, such as diversity combining (maximum ratio combining, equal gain combining, selection combining, etc.), space-time coding or space-time block coding, or multiple input multiple output (“MIMO”).
In a conventional receiver with multiple receive chains, the multiple receive chains are all tuned to the same channel. For instance, conventional MIMO systems may employ multiple antennae and multiple RF chains, such as is shown in FIG. 6. As shown, the system 76 includes multiple receive chains 781, 782, . . . , 78N-1, 78N. Each receive chain is coupled to a respective antenna 801, 802, . . . , 80N-1, 80N. A synthesizer 82, which is fed by a local oscillator LO, is coupled to each of the receive chains. The synthesizer 82 is controlled by the baseband subsystem 84. The baseband subsystem 84 includes respective in-phase and quadrature (“I/Q”) ADCs that are coupled to respective in-phase and quadrature signals of the receiver.
Although the use of multiple receive chains in client terminals improves reception performance, the multiple receive chains consume more power which is not desirable, e.g., for battery operated client terminals. The increased power consumption is caused by the additional receive chains in the RF subsystem and their corresponding ADCs and by increased processing carried out in the baseband subsystem due to additional received signals being received through the additional receive chains.
Most wireless communication systems are designed to operate over a coverage area in which the client terminals are able to communicate with the base station or other network entity. The coverage area can vary from few meters to tens of kilometers. The base station adjusts various signal parameters that include, but which are not limited to, transmit power, channel coding, modulation, number of transmit antennas, etc., so that the transmitted signal can be received successfully by all client terminals including ones that are located in the weak signal area. Often, weak signal areas may be found at the edge of the base station's coverage area, although weak signal areas may be anywhere within the base station's coverage area.
Typically, wireless communication systems operate in an environment that includes the following operational scenarios:
The signal conditions for a client terminal may vary for many reasons. For example, signal conditions may vary depending on the location of the client terminal within the base station coverage area. Also, for a given location, the signal conditions for a client terminal may vary due to changes in the environmental factors, such as due to movement of objects in the surrounding area. Typically, the client terminals estimate the signal conditions by taking measurements. Commonly used indicators of such signal conditions include a Received Signal Strength Indicator (RSSI), a Signal-to-Interference-and-Noise Ratio (SINR), a Bit Error Rate (BER), and a Packet Error Rate (PER), though other indicators may be used to estimate the signal conditions. A client terminal may estimate one or more signal conditions indicators through such measurements.
Normally, certain types of system information are required by all client terminals so that they may communicate with the wireless communication network. The system information typically includes synchronization information, system parameters, allocation information, paging information, etc. The base station must transmit such system information as broadcast data so that all client terminals within its coverage area, irrespective of their capabilities or locations, are able to receive and successfully decode the transmitted information. A worst case scenario may be a client terminal with limited capabilities that is located in a weak signal area. Therefore, the parameters of the base station transmit signal that carries the broadcast data are chosen such that all client terminals in its coverage area may have a high probability of receiving and successfully decoding the broadcast data. This situation often leads to scenarios in which the client terminals that are located in a strong signal area may experience signal conditions that are better than necessary to receive and successfully decode the broadcast data.
Typically, multicast payload data is addressed to a subset of client terminals within a base station's coverage area. Examples of multicast payload data include multimedia information about sports, news, traffic, weather, etc. The base station must transmit the multicast payload data in such a manner that all intended client terminals within its coverage area, irrespective of their capabilities or locations, are able to receive and successfully decode the multicast data. Therefore, the parameters of the transmit signal that carries the multicast payload data are chosen such that all intended client terminals have a high probability of receiving and successfully decoding the multicast payload data. Such choices of parameters often lead to scenarios where the client terminals that are located in a strong signal area experience signal conditions that are better than necessary to receive and successfully decode the multicast payload data.
For user payload data, typically the base station uses power control, link adaptation and other mechanisms to ensure that the signal conditions are optimum for each of the client terminals within its coverage area. However, there are practical limitations to the range of power control, link adaptation, and other mechanisms that the base station may employ. Therefore, despite such mechanisms that the base station may employ, there are some scenarios in which the signal conditions for a particular client terminal may be better than is needed to receive and successfully decode the user payload data.
As wireless communication systems continue to evolve, client terminals with different capabilities may coexist within a given coverage area. Also, some client terminals may support some of the optional features of a wireless communication system whereas other client terminals may not. For example, some client terminals may be designed with four receive chains, some with two receive chains, and others with only one receive chain. The performance capabilities of these client terminals thus varies widely, and yet the wireless communication network must be able to communicate with all of the different types of client terminals, each with its different capabilities, within the coverage area. This requirement inevitably leads to scenarios wherein the client terminals with higher performance capabilities experience signal conditions that may be much better than what the terminals need to receive and successfully decode the data.